This invention relates to the switching of an inductive load by a semiconductor switch, particularly by a MOSFET. Switching arrangements of this type generally include a parallel freewheeling diode for the inductive load--that is, a diode in parallel with the load but oriented to normally conduct current in the opposite direction to the normal polarity of the semiconductor switch. The freewheeling diode protects the semiconductor switch from high voltage transients produced as the switch attempts to abruptly change the current through the inductive load.
This invention is further directed, although not limited, to applications wherein the semiconductor switch handles high currents and is repeatedly activated between its nonconducting and full conducting states, such as in a PWM (pulse width modulation) or duty cycle control of motor operating voltage. Good design of power semiconductor switches in such applications provides for the fastest possible switching in order to minimize the time spent by the semiconductor switch in the active conducting region between the nonconducting and full conducting or saturation conditions and thus minimize heat generation. Minimal heat generation produces lower costs and bulk, since heat sinks can be smaller and devices need not be as insensitive to high temperature.
However, fast switching of inductive loads with freewheeling diodes also tends to generate high radio frequency interference (RFI), since the diode, as it is changing from a reverse biased state to a forward biased state, and vice versa, has a certain recovery time in which it may act as a short circuit. During this time, there is essentially no load resistance for the semiconductor switch and its switching rate is limited mainly by the rate at which current is supplied to its control electrode to charge or discharge its internal capacitance. If this rate is high, sharp voltage transients may be produced in the circuit which are rich in higher harmonics in the radio frequency range. Thus there is a benefit in controlling the switching process over time to obtain a slower switching rate during that portion of the switching process most susceptible to RFI generation and a faster switching rate during that portion of the switching process not so susceptible.
The prior art includes a patent to Bloomer U.S. Pat. No. 4,540,893, issued Sept. 10, 1985, in which the current to the control electrode of a semiconductor switch is made higher below the voltage indicating the threshold of conduction and above the voltage indicating full conduction or saturation of the switch, with a lower current between these voltages through the active conducting region. This patent is not specifically directed to the switching of inductive loads and mentions nothing of the specific difficulties due to the freewheeling diodes used with such loads. In addition, although it appears to have the potential for reducing RFI, it does so by slowing the switching rate during the entire time that the semiconductor switch spends traversing the active conducting region, with consequent higher heat generation, expense and bulk.